Switching AC / DC Relays and

Controlling AC type relays or contactors is tricky and cannot be done
easily with electronics (particularly not with transistors).
In case an electronic control circuit is necessary, a triac or similar
device could be used.
These should preferably be controlled in "zero current" mode, that is, the
control circuit must wait until the current is zero and not the voltage of
the AC supply. (90 degrees lagging current)

There are (or were), special ICs for this application.

If no zero current mode is possible nor used, then a snubber circuit should
be connected in parallel with the coil, but still it is tricky business.
(the Triac typicaly will not switch off reliably or at all, and also could
be damaged). A snubber must contain a resistor AND a capacitor in series
(rather difficult to design, except by trial and error).

I have never seeen shunt (parallel) resistor with AC contactor coils to
improve the switching, though who knows, I may be getting too old.

Another comment, in case of DC powered coils and wheel diodes:

Circuit theory easily shows that the maximum current that an inductance
will try to pass through an external circuit is equal (and NOT higher) to
the current that was circulating on it immediately before the power is
switched off.

I don't like analogies, but as an exception: you don't expect your car to
jump into hyperspace like the Enterprise when you clutch suddenly; it will
try to keep its previous speed for a while and (more or less) quickly slow
down.

Inductances are similar. So no risk of overloading the wheel diode if it is
rated at least for the normal operating current of the relay coil.

Note that we a re lucky enough or Nature is so wise, as to have the voltage
on the inductance reverse its polarity (Lenz law) upon switch off, so that
we can connect our diodes without polarity problems.

But the real problem is that if you do not provide an alternative path to
the current the inductors try to maintain after being switched off, the
indutance will try to overcome the resistance by proportionally increasing
the voltage on its terminals (the infamous spike that burns any transistor).

ANY resistance on the circuitt (such as the "shunt" resistance we are
talking about) can only increase the voltage over the original value
applied to the inductance.

That is, simply put, why diodes are the best solution in DC circuits (they
have practically zero resistance in the conducting state).

Regards,

Jose Luis
At 09:25 05/10/2001 -0400, you wrote:
>Mark,
>I wasn't quite "with it" yesterday, and didn't think your question through
>completely.
>A "clamping" or "wheel" diode across a relay's coil works great *IF* the
>voltage applied to the coil is DC. However, this won't do for AC, as the
>clamping/wheel diode would present a dead short to ground for 1/2 off the
>cycle (when the AC voltage went negative).
>
>For this reason, in AC circuits only, you must use a shunt resistor across
>the relay's coil (or other inductive load).
>
>
Original Message
>From: Mark Adams [noparse][[/noparse]mailto:madams@a...]
>We were looking at some BOSCH relays and they listed an option that
>included a 'shunt resistor' and the ones we currently purchase with an
>included diode. Now, the question is why is the shunt resistor there?
><snip>
>
>
>
>To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
>from the same email address that you subscribed. Text in the Subject and
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[noparse][[/noparse]Non-text portions of this message have been removed]

Comments

Switching highly inductive AC relay coils (large contactors) is very
easy to do. Most Solid State Relay modules do employ zero switching.
An appropriatly sized SSR can easily handle the demands of a
contactor coil. Also, a transistor can drive a small DC relay, which
can then switch the contactor. Finally, the difference between an AC
coil and a DC coil is that the AC coil has a "chatter" ring on the
coil. A coil is after all a coil. Almost any electromechanical relay
with an AC coil can be operated by DC. A relay with a DC coil wont
work well with AC due to the absence of the chatter ring on the coil.
Over the years I have driven many AC coil relays with DC with no ill
effects.

Precautions should be taken to assure that the power supplying
sensitive electronics is well filtered and regulated. I almost always
use .1uF caps across Vdd and Vss pins

My two cents

Regards
rich

--- In basicstamps@y..., Jose Luis Juarez <jl.ingenieria@i...> wrote:
> Still some comments:
>
> Controlling AC type relays or contactors is tricky and cannot be
done
> easily with electronics (particularly not with transistors).
> In case an electronic control circuit is necessary, a triac or
similar
> device could be used.
> These should preferably be controlled in "zero current" mode, that
is, the
> control circuit must wait until the current is zero and not the
voltage of
> the AC supply. (90 degrees lagging current)
>
> There are (or were), special ICs for this application.
>
> If no zero current mode is possible nor used, then a snubber
circuit should
> be connected in parallel with the coil, but still it is tricky
business.
> (the Triac typicaly will not switch off reliably or at all, and
also could
> be damaged). A snubber must contain a resistor AND a capacitor in
series
> (rather difficult to design, except by trial and error).
>
> I have never seeen shunt (parallel) resistor with AC contactor
coils to
> improve the switching, though who knows, I may be getting too old.
>
> Another comment, in case of DC powered coils and wheel diodes:
>
> Circuit theory easily shows that the maximum current that an
inductance
> will try to pass through an external circuit is equal (and NOT
higher) to
> the current that was circulating on it immediately before the power
is
> switched off.
>
> I don't like analogies, but as an exception: you don't expect your
car to
> jump into hyperspace like the Enterprise when you clutch suddenly;
it will
> try to keep its previous speed for a while and (more or less)
quickly slow
> down.
>
> Inductances are similar. So no risk of overloading the wheel diode
if it is
> rated at least for the normal operating current of the relay coil.
>
> Note that we a re lucky enough or Nature is so wise, as to have the
voltage
> on the inductance reverse its polarity (Lenz law) upon switch off,
so that
> we can connect our diodes without polarity problems.
>
> But the real problem is that if you do not provide an alternative
path to
> the current the inductors try to maintain after being switched off,
the
> indutance will try to overcome the resistance by proportionally
increasing
> the voltage on its terminals (the infamous spike that burns any
transistor).
>
> ANY resistance on the circuitt (such as the "shunt" resistance we
are
> talking about) can only increase the voltage over the original
value
> applied to the inductance.
>
> That is, simply put, why diodes are the best solution in DC
circuits (they
> have practically zero resistance in the conducting state).
>
> Regards,
>
> Jose Luis
> At 09:25 05/10/2001 -0400, you wrote:
> >Mark,
> >I wasn't quite "with it" yesterday, and didn't think your question
through
> >completely.
> >A "clamping" or "wheel" diode across a relay's coil works great
*IF* the
> >voltage applied to the coil is DC. However, this won't do for AC,
as the
> >clamping/wheel diode would present a dead short to ground for 1/2
off the
> >cycle (when the AC voltage went negative).
> >
> >For this reason, in AC circuits only, you must use a shunt
resistor across
> >the relay's coil (or other inductive load).
> >
> >
Original Message
> >From: Mark Adams [noparse][[/noparse]mailto:madams@a...]
> >We were looking at some BOSCH relays and they listed an option that
> >included a 'shunt resistor' and the ones we currently purchase
with an
> >included diode. Now, the question is why is the shunt resistor
there?
> ><snip>
> >
> >
> >
> >To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@y...
> >from the same email address that you subscribed. Text in the
Subject and
> >Body of the message will be ignored.
> >
> >
> >Your use of Yahoo! Groups is subject tohttp://docs.yahoo.com/info/terms/
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]

Another possibility for switching an AC load is to put it in series
with the input (AC) terminals of an appropriately rated full-wave
bridge. Your switching transistor goes inside the bridge and sees
only DC. Comments about transients still apply. If the control
circuit doesn't require much current, you can often develop a
half-wave supply for it, with a separate rectifier off the same AC
supply referenced to the (-) side of the bridge.

This is works best for fairly small loads with control circuits that
don't have any opportunity for user contact, like fully-enclosed
appliance timers etc. since it is NOT isolated from the power line.